Radiation thought to be from Japan has been detected in minute amounts in the United States, including in Washington state.

Minuscule quantities of the radioactive isotope xenon-133 were detected by sensors in Washington state Wednesday and Thursday and in Sacramento, Calif., on Friday, the Environmental Protection Agency and federal Department of Energy said in a joint statement issued Friday.

“The origin was determined to be consistent with a release from the Fukushima reactors in northern Japan,” the statement said.

The levels detected were about 0.1 disintegrations per second per cubic meter of air — written as 0.1 Bq/m3 — the statement said.

That amount “results in a dose rate approximately one-millionth of the dose rate that a person normally receives from rocks, bricks, the sun and other natural background sources.”

The statement defined xenon-133 as a radioactive noble gas produced during nuclear fission “that poses no concern at the detected level.”

“These types of readings remain consistent with our expectations since the onset of this tragedy and are to be expected in the coming days,” the statement said.

After the explosion of the Chernobyl plant in Ukraine in 1986, which spewed far more radioactive material higher into the atmosphere than the damaged Japanese reactors, air monitors in the United States picked up trace amounts of radioactive particles that were less than one thousandth of the estimated annual dose from natural sources for a typical person.

No plume

In the wake of the discovery, many national news agencies referred to a “radioactive plume” hitting the United States.

“We’re not aware of anything we would call a plume of radiation,” said Tim Church, communications director for the state Department of Health, on Friday.

“There’s been unsubstantiated national reports, but no one in any position of expertise has used that term.”

On Tuesday, the Comprehensive Test Ban Treaty Organization, an arm of the United Nations in Vienna, released a model that predicted how a plume of radiation from Japan could disperse worldwide if there were a sustained release of radiation high into the atmosphere.

A large and continuous release would be necessary to spew radioactive particles so high that the jet stream picked them up.

“This has not happened yet,” wrote Cliff Maas, professor of atmospheric sciences at the University of Washington, on his blog, www.cliffmass.blogspot.com, on Friday.

“Measurement by the U.S. assets during the past day have shown that significant radiation has not spread beyond the immediate facility,” Maas wrote.

“Even if radiation got to the jet stream, we think the most we would see here are small amounts that are no concern to public health,” Church said.

“By the time it traveled 5,000 miles, it would be so dispersed that it would not be harmful to anybody,” he added.

Church said the state, along with the EPA, is monitoring radiation levels in Washington state.

“We at the state level continue to look for anything that shows we have an increase of radiation levels,” he said.